How climate researchers can look back 15,000 years
The ice in the poles harbors few microbes, and the annual growth rate is often high. It is therefore usually easier to date climate changes there than with non-polar glaciers. But researchers have now succeeded in making a spectacular measurement on just such an ice cap in Tibet.
Bear cores milled from glaciers and the ice caps of the poles are something like time machines – climate archives spanning thousands of years. A team of researchers has now looked back at 15,000 years of regional climate variability for the Guliya Ice Cap, a glacier in the Tibetan Highlands in western China. The result was published in the journal “Proceedings” of the National Academy of Sciences in the USA (“PNAS”): Accordingly, the data indicate a clear climate change in the region.
Outside of the polar regions, the Tibetan highlands – also known as the roof of the world – contain the largest frozen freshwater reservoir on earth, as explained by the scientists led by Lonnie Thompson from the Ohio State University in Columbus. The glaciers and ice fields of the plateau and the adjacent Himalayan mountains therefore contribute significantly to the water resources of the region, in which a quarter of the world’s population lives. Agriculture, municipal water supply and hydropower in countries in South and Southeast Asia depend on it.
Climate change could have a correspondingly devastating effect in the region, causing glaciers and ice fields to melt. “The high-altitude glaciated areas of Asia are warming faster than the global average, and under the 1.5 degree global warming scenario, glaciers in the mountains bordering the Tibetan Plateau are expected to lose half their surface area by 2100. “
The key is in the bubbles
In the northwestern Tibetan highlands, the air masses of the Indian summer monsoon and the continental westerly winds interact. There, in the western Kunlun Mountains, the Guliya Glacier is about 6000 meters above sea level. According to the research team, dating ice cores from glaciers on the plateau is difficult, partly because comparatively little ice is added there each year and because microorganisms influence certain values.
The researchers now dated the ice from three cores about 50 meters deep using the composition of atmospheric oxygen in air bubbles (Delta O-18 value), radiocarbon dating of plant fragments in the ice and annual layer counts. The data obtained on this basis therefore corresponded to existing records, for example from lakes.
If the patterns of evaporation and precipitation in a region change as a result of climate change, the so-called delta-O-18 ratio of oxygen changes. Oxygen (O) is a mixture of three natural so-called isotopes: O-16, O-17 and O-18. The respective composition in a region depends on the temperature – partly because water with the heavy isotope O-18 incorporated evaporates less quickly than water with the lighter isotope O-16. For drill core analyses, this means that ice poorer in O-18 comes from a period of colder climates.
The delta O-18 value indicates the O-18O/O-16 abundance ratio and allows direct conclusions to be drawn about the respective temperature in the period in which the examined ice sample was formed. The Delta-O-18 method has often been used to draw conclusions about the temperature development in ice cores from the poles. Thompson’s research team has now successfully adapted it for use on highland glaciers.
The average delta O-18 value from 1950 to 2014 was the highest since almost the beginning of the Holocene about 12,000 years ago. This is consistent with the finding that the global mean surface temperature is currently the highest it has been in more than 11,000 years. The developed method should also be useful for paleoclimate records from other high-altitude, non-polar glaciers, according to the research team.
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